I have taken apart my 2018 MacBook Pro and there is no glue and it's no more difficult to repair than any of the other models. Provided you can get the parts of course.
Also the A1278 is at best a 6 year old machine. I would imagine it's pretty normal for there to be some wear/tear issues.
I have an IBM X60 from 2006, and the only problem it's developed despite heavy (mobile) use for the past decade-and-nearly-half is that it's lost some of the paint on the case around the edges and bottom.
A friend's MacBook (the plastic ones of the time) started cracking its case within a year, and shortly after that the HDD failed.
Apple has always presented their products as being premium, and their aesthetic isn't bad, but IMHO they're not really as well-built as they claim to be. They are better built than the cheap HPs and Toshibas, however.
Also the A1278 is at best a 6 year old machine. I would imagine it's pretty normal for there to be some wear/tear issues.